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The relationship between endotoxin contamination of cotton dusts and pulmonary function was studied in humans. Forty three human subjects were exposed to clean air or dusts generated by carding from three varieties of cotton. The exposure period was 6 hours. Cotton grown in California, Texas, and Mississippi was used. Forced expiratory volume in one second (FEV1) was measured immediately before and after exposure. The concentration of gram negative bacterial endotoxins in dust samples was determined. Cotton dust concentrations were measured gravimetrically. Attempts were made to correlate pulmonary function with endotoxin and cotton dust concentrations. Endotoxin concentrations in the dusts varied markedly depending on the variety of cotton and the geographical area where it was grown. Cotton grown in Mississippi had the highest average endotoxin concentration, 87.60 to 164.89 nanograms per cubic meter (ng/m3), while cotton grown in Texas had the lowest average endotoxin concentration, 10.63ng/m3. There was less variation in dust concentration with geographical area or cotton variety. Average dust concentrations for all cotton samples ranged from 0.27 to 0.50 milligram/m3. All cotton dusts decreased FEV1 performance. The decreases in FEV1 values were strongly correlated with endotoxin content of the dusts, but not with cotton dust concentration. The authors conclude that the variety of cotton and where it is grown affect the endotoxins of cotton dust. Acute pulmonary function changes correlate well with endotoxin concentrations.